Abstract

High-pressure Raman measurements of type-III germaniumclathrate have been made up to 26 GPa at room temperature. We observed low-frequency vibrational (rattling) modes associated with guest Ba atoms at , and host Ge framework vibrations around , which are compared with those of silicon clathrate. High-pressurephase transition was found at 3.2 GPa, which seems to be due to the structural distortion combined with the enhanced guest-host interactions. becomes irreversibly amorphous at pressures above 22 GPa. This pressure is less than that of type-I clathrate, indicating that type-III structure is less stable than type-I Geclathrate under high pressures.